Valve spring assemblies for automotive engines comprise a valve having a long stem with an annular groove near the end, a coil spring around the stem, a cap over the end of the spring, and a pair of semi-annular keys which seat in the groove and hold the cap against the spring while the cap, in turn, presses the keys into the groove by way of a tapered surface. These assemblies are put together by automatic equipment and then inspected for proper fit. To accomplish key insertion, the spring must be compressed so that the keys can lock into the valve stem groove. When the spring is released, the keys hold the spring in place. If one key should pop out or jam in a position other than in the notch, the other key will temporarily hold the assembly together, but will release when the valve is exercised. The inspection must detect any missing key to avoid that problem. Even if a key is not missing, it may not be seated properly. Thus a high key is a symptom of improper seating and must be detected.
Automatic inspection of the valve spring assemblies has been done by a machine vision system as shown in the U.S. Pat. No. 4,399,554 to Perkins III et al wherein a camera positioned above the engine can view the valve assembly and the scene is analyzed by a computer program to locate the stem and the keys surrounding the stem. A missing key is readily detected. The vision system is not sensitive to small variations in range or key height so that in practice another camera is positioned to capture a side view of the keys (which normally extend above the cap) and detect improper key height. However, some valve assemblies have the keys recessed in the cap so that the side camera can not see the keys to verify the height. Also dual contact probes coupled to LVDTs and mounted on a robot end effector are in use to make a differential measurement between the top surfaces of the stem and keys. These probes are unable to detect a missing or low key if the other key is in the stem groove.
It is thus proposed to use a sensing system which is sensitive to key height as well as to key presence so that the entire inspection can be carried out by a single sensor above the valve assembly. Ultrasonic inspection is proposed since it is based on range measurements. As shown in U.S. Pat. No. 4,554,834 to Prinz et al, an ultrasonic transducer carried by a robot can be used to map the profile of a workpiece using a focused sonic beam. Also, U.S. Pat. No. 4,332,016 to Berntsen uses ultrasonic measurements to determine characteristic three dimensional dimensions of objects which are carried past the measuring device. While it is known that ultrasonic measurements are subject to temperature effects due to the temperature dependence of the speed of sound, these patents do not reveal how to make accurate measurements in an environment where temperature may vary.